Hard disc drives are commonly used as the primary data storage and retrieval devices in modern computer systems. In a typical disc drive, data are magnetically stored on one or more discs that are rotated at a constant high speed and accessed by a rotary actuator assembly having a plurality of read/write heads that fly adjacent the surfaces of the discs.
The rotary actuator assembly further includes a coil of a voice coil motor, so that the heads are positioned relative to the discs in response to the application of current to the coil. A read channel and an interface circuit, responsive to the heads, are provided to transfer previously stored data from the discs to the host computer.
A closed loop digital servo system such as disclosed in U.S. Pat. No. 5,262,907 issued Nov. 16, 1993 to Duffy et al., assigned to the assignee of the present invention, is typically used to control the position of the heads relative to tracks on the discs. The tracks are defined from servo information that is written to the surfaces of the discs during manufacturing. The servo system of a disc drive thus utilizes the servo information in the performance of two primary operations: seeking and track following.
Generally, a seek operation entails the movement of a selected head from an initial track to a destination track. A velocity-control approach is typically employed wherein the velocity of the head is measured and compared to a velocity profile which defines an optimum velocity trajectory for the head as it moves to the target track. The velocity profile is typically arranged as a series of target velocities in terms of the number of tracks remaining in the seek; that is, the number of "tracks to go" until the target track is reached. Thus, during a seek the location and velocity of the head is repetitively determined and current is applied to the actuator coil in proportion to the difference between the measured velocity and the target velocity of the head.
Track following entails the continued positioning of a head over a corresponding, selected track. A position-control approach is typically employed wherein the relative position of the head with respect to the center of the track is determined and compared to a desired position for the head (whether over the center of the track, or a selected percentage of the width of the track away from the center of the track). A position error signal (PES) indicative of position error is generated therefrom and used to control the amount of current that is applied to the actuator coil in order to maintain the head at the desired position relative to the track.
It will be recognized that in both track following and seek modes of operation, it is essential that the servo system accurately determine the address of the track over which the head is located. During a seek operation, for example, misidentification of the track address can lead to errors in the trajectory of the head, potentially resulting in the head settling upon the wrong track (and requiring additional time for the head to be positioned over the intended, destination track). Similarly, during track following, misidentification of the track being followed can result in the declaration of a servo fault, causing a suspension of a read or write operation until the head is determined to be over the proper track.
As is known in the art, each of the blocks of servo information written to the discs typically includes a track address field in which track address information is stored. It is common to encode this track address information in the form of Gray code (GC), which generally comprises a multi-bit sequence, with each value in the sequence corresponding to a physical track address. Unlike a typical binary sequence, however, only one bit in the GC sequence changes as the head moves to each adjacent track, in order to facilitate improved track address detection. Accordingly, each track is assigned a unique GC value and the total number of GC bits in each value is selected to accommodate the total number of tracks on each disc.
When a head reads a track address field (also referred to as a "Gray code" or "GC" field), an analog GC readback signal is obtained having both positive and negative peaks, as well as baseline portions disposed between adjacent peaks, with the baseline portions ideally having an amplitude near zero. The readback signal is compared to both positive and negative detection thresholds and digitized into either ones or zeros, depending upon whether the signal is within or beyond the thresholds. A servo demodulator then checks the resulting digitized value against the value that the GC field should generate to ensure the head is over the intended track. Should an error in the resulting GC value be detected, the servo system will declare a fault condition and attempt to adjust the location of the head so as to bring the head in proper relation to the disc.
As will be recognized, a continuing trend in the industry is to provide disc drives with ever increasing data storage capacities and transfer rates. Some disc drives of the present generation have track densities greater than 3000 tracks/centimeter (8000 tracks/inch). With the use of such increased track densities, as well the use of other advances in the art such as magneto-resistive (MR) heads, modern disc drives have increasingly encountered two types of problems when decoding GC readback signals: 1) baseline shifting and 2) signal amplitude asymmetry.
Baseline shifting, or shouldering, is characterized by a positive or negative shift in the amplitude of the baseline portion of the signal between adjacent peaks so that the GC readback signal takes non-zero values at times when a corresponding ideal signal should be close to zero. When such shifting is sufficiently pronounced, the amplitude of the GC readback signal can rise above or below the positive and negative detection thresholds, respectively, resulting in the baseline portion being erroneously detected as a peak.
Signal asymmetry is characterized by differences between the maximum positive peak amplitude and the maximum negative peak amplitude in a GC readback signal. Such asymmetry can result in misdetection of portions of the readback signal. For example, the positive threshold may be of adequate magnitude to properly detect the positive peaks in the readback signal, but using an equal opposite magnitude for the negative threshold may result in a value that is too close to the baseline to ensure reliable negative peak detection. Conversely, if the positive or negative peak amplitudes in the GC readback signal are sufficiently low, such peaks will not be reliably detected when the magnitudes of the corresponding thresholds are too great.
Accordingly, there is a need in the art whereby disc drive GC readback signals can be reliably decoded in order to accurately determine head location in the drive, while accommodating continued advancements in the art which tend to degrade the GC readback signals by introducing signal asperities such as baseline shifting and signal amplitude asymmetry.